Security documents such as banknotes now frequently carry optically variable devices such as diffraction gratings or holographic optical microstructures as a security feature against copy and counterfeit. This has been motivated by the progress in the fields of computer-based desktop publishing and scanning, which renders conventional security print technologies such as intaglio and offset printing more prone to attempts to replicate or mimic. Examples of such holographic structures and their manufacturing techniques can be found in EP0548142 and EP0632767 filed in the name of De La Rue Holographics Ltd.
The use of diffraction gratings or holographic optical microstructures has become more prevalent in recent years and consequently the underlying component technologies/sciences have become increasingly accessible to would be counterfeiters.
Optically variable devices can also be created using non-holographic micro-optics. One advantage is that mechanical copying of micro-optical components, such as spherical or cylindrical microlenses, typically with a size range of 1-50 μm, is very difficult to achieve because any variation in dimension or geometrical distortion leads to a decline or extinction of the required optical properties. Arrays of cylindrical lenses in combination with interlaced image strips have been routinely used in the packaging and advertising industries to create autostereoscopic and dynamic images. These are known in the industry as lenticular images.
Security devices utilising lenticular images have been described in the literature. U.S. Pat. No. 4,892,336 describes a security thread which has a printed pattern on one side and a lenticular structure coordinated with the printed pattern on the other side which combine in such a way that when the document is turned about an axis parallel to the cylinder lenses, a motif moves almost continuously from one place on the security thread to another.
The prior art discussed above uses cylindrical lens arrays. The use of cylindrical lenses limits the optically variable nature of the device in that the associated movement or change of image only occurs when the viewpoint rotates about the long axis of the lens.
Lenticular images are further limited by the requirement for precise register between the microlenses and the printed images and they are therefore very difficult to manufacture using mass production techniques, which provides a barrier to using them commercially.
A practical problem with lenticular devices is that the thickness is dependent on the width and number of the interlaced image strips. Referring to FIG. 1 in order for the device to function the back focal length, f, of the lens 10 must be such that it focuses on the image strips A,B,C and the repeating period, p, of the image strips must be the same as the lens diameter, D. The back focal length 11 of the lens is defined as the distance from the back surface of the lens to the focal point 12. As a general guide for polymer films f˜≧1−1.5×D. Therefore for a device to be 30 μm thick the lens diameter must be no greater than 30 μm. Consequently, the repeat period for the image strip would have to be no more than 30 μm. This is not practical with conventional printing techniques such as gravure, lithography and intaglio which can at best achieve resolutions of 20 μm/pixel correlating to ˜1200 dpi. Commercially available lenticular devices are therefore relatively thick (>150 μm) and this has prevented their use on/in flexible security documents such as banknotes where devices typically have thicknesses in the range 1-50 μm.
The use of moiré magnifiers for the creation of security devices is known. U.S. Pat. No. 5,712,731, filed in the name of De La Rue International Limited, discusses that combinations of microlenses and microimages can be used to generate security devices exhibiting optically variable effects. In the simplest case of a small pitch mismatch between the lens arrays and image arrays, an array of magnified images of constant magnification is observed with motion resulting from the normal parallax of a lens. A pitch mismatch between a microlens array 14 and a microimage array 13 can also conveniently be generated by rotating the microimage array 13 relative to the microlens array 14 or vice-versa, such that the microlens and microimage array have a rotational misalignment, as shown in FIG. 2. The rotational misalignment or the small pitch mismatch results in the eye observing a different part of the image in each neighbouring lens resulting in a magnified image 15, illustrated in FIG. 2 for the case of rotational misalignment. If the eye is then moved relative to the lens/image array a different part of the image is observed giving the impression that the image is in a different position. If the eye is moved in a smooth manner a series of images are observed giving rise to the impression that the image is moving relative to the surface. In the case where the pitch mismatch is generated by rotational misalignment the array of magnified images is rotated relative to the microimage array and consequently the parallax affect that results in the apparent movement of the magnified image is also rotated and this is known as skew parallax. The effect of pitch mismatch and rotational misalignment on the magnification and rotation of the magnified image observed in a moiré magnifier is described in “The Moiré Magnifier”, M. Hutley, R Hunt, R F Stevens and P Savander, Pure Appl. Opt 3 (1994) 133-142 published by IOP Publishing Limited.
The nature of the movement and orientation changes can be explained from the theory of moiré; this is discussed in detail in “The theory of the Moire phenomenon” by I. Amidror published by Kluiver Academic Publishers in 2000, ISBN 0-7923-5949-6. The moiré effect of two periodic structures can be explained/predicted by considering the frequency vectors of the two structures. The orientation of the frequency vector represents the direction of the periodicity and the length represents the frequency (i.e. 1/Period). The vector is expressed by its Cartesian coordinates (u,v) where u and v are the horizontal and vertical components of the frequency.
A one-dimensional grating is represented by a pair of points in the frequency plane (strictly, the grating should be a sinusoid to only have two points in the frequency plane). The representation of two one-dimensional gratings, with the same frequency but different orientation as frequency vectors, is shown in FIG. 3. Grating 1 can be represented by the two points f1 and −f1 and grating 2 can be represented by the two points f2 and −f2. The spectrum produced from the convolution of the two frequency representations (grating 3) indicates what moiré frequencies will occur. To be visible, the moiré must be close to the origin (i.e. a low frequency/high period), so the moiré observed from the superposition of the two gratings in FIG. 3 corresponds to f1-f2 and f2-f1 in the frequency plane. For two-dimensional rectilinear gratings, the same principle applies but occurs in the two orthogonal directions at once.
A large magnification corresponds to a low frequency moiré. It can be seen from the frequency representation in FIG. 3 that a low frequency moiré requires a close match of frequency and orientation. It can also be seen in FIG. 3 that the resultant moiré frequency vector is at ˜90° to the individual frequency vectors. If instead of two one-dimensional gratings we have a superposition of a microlens array and a microimage array then the resultant moiré frequency vector corresponds to the magnified image array which will be orientated at ˜90° to the microlens and microimage arrays. The degree of magnification is dependent on the ratio of the frequency of the microimage over the frequency of the magnified (moiré) image, i.e. fmicroimage/fmoire. In the same manner that the orientation of the magnified image is rotated, the parallax affect that results in the apparent movement of the magnified image is also rotated. In this condition tilting the combined lens/image array vertically about the horizontal axis results in a counter intuitive horizontal motion in the magnified image. This is known as ortho-parallactic motion, i.e. motion perpendicular to the normal direction of the parallax and has been discussed in relation to security devices in a conference paper by Nanoventions, Inc in Proc. of SPIE-IS&T Electronic Imaging SPIE Vol. 5310 p 321-327.
U.S. Pat. No. 5,712,731 discloses the use of the microlens array as a separate device for viewing the associated microimage or as one fully bonded device. However it is not possible to produce microimages at a sufficiently small scale to create a thin (<50 μm) fully bonded flexible device using the printing or embossing techniques disclosed in U.S. Pat. No. 5,712,731.
An alternative approach to forming high-resolution black and white images is described within U.S. Pat. No. 5,503,902 filed in the name of Applied Physics Research, L.P., and specifically in relation to security devices in patent publication US 20030179364. The images are constructed by using light traps to create the black pixels. The light traps comprise reflective tapered structures with relatively high aspect ratios. Light entering the tapered structure is reflected and approximately 10% of the light may be absorbed. Due to the morphology of the structures the light is reflected many times and in this way all of the light is absorbed by the structure before it has the chance to exit the light trap. The light traps are formed during reactive ion etching of a layer of photopolymer and are a result of impurities/inclusions in the composition of the photopolymer. A master is then generated using conventional photolithographic techniques and the structure is then replicated into a polymer film. This technique allows the creation of black pixels at the limit of optical detection with a resolution of up to 100000 dpi. Arrays of microimages generated from such very fine pixels can be combined with microlenses to form thin flexible security devices of <50 μm producing autostereoscopic and dynamic images. A high-resolution colour image can be created by overprinting the high-resolution black and white image with a transparent lower resolution colour image.
Although the light traps allow the formation of very high resolution identifying images there are a number of processing issues associated with them. The light traps are formed during reactive ion etching as a result of impurities or inclusions in a photopolymer. Therefore the exact structure of each light trap cannot be specified and is not reproducible which could lead to variability in the final image. Once the structures are formed and a master produced the structure must be copied with very high fidelity into polymeric film using a replication process suitable for mass production. The high aspect ratio of the light trap structures (˜1:5) can result in difficulties in replicating these structures in polymeric films. It is known that the fidelity of replicated nanoscale and microscale structures is very dependent on the aspect ratio. In “Micro-optics. Elements, Systems and Applications—Edited by Herzog”, it is reported that an aspect ratio of 1:1 can be replicated easily, 1:5 with care and 1:10 only with great difficulty. Diffractive structures used in security holograms have aspect ratios of ˜1:1 and are routinely produced at very high production rates (60 m/min) using a continuous hot embossing process. High aspect ratio structures cannot be replicated accurately using this process. UV embossing can be used to replicate such high aspect ratio structures but as the aspect ratio increases the slower and more difficult the process becomes. For a security device to be useful commercially genuine devices must be relatively easy to manufacture since otherwise production cost would be prohibitive. There is therefore a requirement for high-resolution images with high contrast that can be formed from structures that can be replicated with a high fidelity using a commercially cost effective process.